C#: Add test for local function in lambda

Author: hvitved

csharp/ql/test/library-tests/csharp7/CSharp7.cs

@@ -148,9 +148,9 @@ int f8()
             return f9(1);
         }
 
-        {
+        Action a = () => {
             int f9() => 0;
-        }
+        };
 
         return f1(2);
     }